1. Field of the Invention
This invention relates to photomultiplier tube structures and, more particularly, to a novel dynode structure for a photomultiplier tube suitable for use with a low-light scintillator.
2. Background of the Prior Art
Many situations require precise measurements of the photon outputs from sources of gamma rays, x-rays, low level visible light and the like. In principle, a quantum of energy from a source of interest is received at a photocathode and causes the emission of electrons therefrom. This initial supply of electrons reaching a photocathode normally is too small to permit practical measurements. Controlled amplification of this initial electron emission with the minimum amount of "noise" in the process is highly desirable, and is provided by a photomuliplier tube.
Typically, the incident energy is received at a semitransparent thin cathode film in the photomultiplier tube which receives incident radiation on one side and emits a corresponding output of electrons from its other side, inside the photomultiplier tube. These photoelectrons are focused or guided by electrostatic or electromagnetic fields to the active surfaces of a set of dynodes. Where electron flow is controlled by electrostatic fields, one or more grids may be provided between successive dynodes to contain the flow of electrons. Finally, the amplified output of electrons is received for analysis and interpretation in known manner.
A major consideration in the design of any photomultiplier tube is how to shape and position the dynodes, so that all of them are properly utilized and no electrons are lost to support structures within the photomulitplier tube or deflected in other ways. It is not necessary that the electrons from each dynode or stage come to a sharp focus on each succeeding stage. The force fields, however, should control the electron flows to reach a generally central location on each receiving dynode. This receiving portion of the dynode surface then becomes the emission point for the next stage. It is not essential for efficient operation of the photomultiplier tube that the emission point be located at the optimum location of each dynode. However, if the flow of electrons received by each dynode is not adequately controlled, electrons tend to increasingly diverge from the center of each successive dynode and this, in turn, may lead to the skipping of successive stages by the flow of electrons with consequent loss of amplification gain.
It is very important to provide good collection of secondary electrons from stage to stage within the photomultiplier tube. It may also be important to minimize the time-spread of electron trajectories from the first to the last dynode. One solution is to provide strong electric fields at the surfaces of the dynodes to assure high initial acceleration of the electrons. It is usually also important to design the dynode surface configurations to thereby provide nearly equal transit times between dynodes regardless of the point of emission of electrons on a given dynode.
In utilizing such photomultipliers, relatively large photocathode areas may be required for efficient scintillator coupling. For a relatively large photocathode, the first problem is to design an efficient "photocathode to first dynode" electron flow. Failure to guide the photoelectrons properly through the first dynode will result in a high noise-to-signal ratio and this will result in poor pulse-height resolution characteristics. Regenerative effects, i.e., an open path in the tube through which occasional ions or light can feed back from the output end to the photocathode, must be avoided.
A number of known photomultiplier structures are described in the RCA photomultiplier handbook (1980 Edition), particularly at pages 26-35 thereof, and these include the structures illustrated in FIGS. 1, 2, and 3 of this application.
FIG. 1 illustrates a known photomultiplier tube structure 20 with a front window 22, a semitransparent photocathode 24 to generate electron flow 26, a first electron focusing element 28 to apply a skewed focusing electrostatic field to the photoelectron flow 26, and a first shaped dynode surface 30 to receive the focused photoelectron flow. Secondary emission 32 of electrons flows in typically highly curved paths from first dynode 30 to a second dynode 34, the receiving surface of which is capable of generating a corresponding amplified emission of electrons received in succession by other dynodes such as 36, 38 (and the like) that are arranged in a circular "cage" manner to end with an anode 40 coupled to an electrical circuit for analyzing the amplified output of the incident energy flow. As seen from FIG. 1, particularly between the first dynode 30 and the second dynode 34, the paths follows by successively generated electrons between adjacent dynodes are not all of the same length. Also, because of the distribution of electrostatic field strength between dynodes, electrons are not all equally accelerated in their passage through the dynode cage. The actual shapes selected for the dynodes have a strong influence on the paths taken by the successive electron flows and hence on the disparity between the time taken for the results of photoelectron emissions from various parts of the photocathode to reach ceiling electrode 40. The various electron paths are very complex and, consequently, it is a major problem to correctly shape such dynodes.
Incident radiation enters the photomultiplier tube of FIG. 2 at a front surface 62, to reach a semitransparent photocathode 64 which emits photoelectrons 66 guided by a focusing electrode 68 to an active surface of first dynode 70. An emission of electrons 72 from the first dynode 70 then reaches the active surface of a second dynode 74, and so on through a plurality of similar dynode surfaces to reach collecting electrode 80. Suitably charged grids, such as 76 for the first dynode and 78 for the second dynode, are provided with each dynode to reduce the noise-to-signal ratio by guiding the successive electron flows between dynodes to reduce straight transfer of electrons within the photomultiplier tube 60. Such a structure is known as a "box-and-grid" structure and has individual dynode boxes, each open at the exit end and having a grid at its entrance. An electric field generated by the grid penetrates the preceding dynode region and aids in the withdrawal of secondary electrons, while also eliminating a retarding field that would be caused by the potential dynode. Therefore, because the field penetration is rather weak, the electron transit time between dynodes of the box and grid type is relatively slow and has a rather large time spread.
Another known photomultiplier tube structure 100, of the type known as a "venetian-blind" structure, is illustrated in FIG. 3. This structure is very flexible as to the number of stages and, like the previously discussed "cage" and "box-and-grid" structures, has a front window 102 through which energy reaches a photocathode 104 to generate emission 106. A focusing electrode 108 directs the flow of photoelectrons 106 to first dynode surfaces 110 from which a secondary emission of electrons 112 flows to second dynode surfaces 114. Control grids 116 for the first dynode, 118 for the second dynode, and the like are provided for by a plurality of grids between the first dynode and the receiving electrode 120. This venetian-blind structure is flexible as to the number of stages that may be provided. However, it is rather slow in response time and some of the secondary electrons may be lost because of the interposition of the grids between stages, as was the case with the "box-and-grid" structure illustrated in FIG. 2.
FIGS. 1, 2 and 3 are generally schematic, and focus on the electron paths rather than on details of the structure. FIG. 4, however, illustrates certain details of venetian-blind dynodes. Thus, photomultiplier tube 130 has a front window 132, a semitransparent photocathode 134, and a first grid 136 associated with a first dynode surface having venetian blinds 140 formed in a generally cup-like shape with a rim portion 144 held by attachment to a circular flange piece 146. Successive venetian-blind elements are held adjacent each other, e.g., 140 and 142, each with their respective grids 136 and 138 attached. Adjacent elements are separated by ceramic elements 148, and the tube ends in a rear surface 150. The shape of the typical venetian-blind structure is better understood with reference to FIGS. 5 and 6. The venetian-blind structure, comprising the blinds 140 and the rim 144, must be made of a material capable of generating the desired secondary emissions and must be combined with other elements of the photomultiplier tube in a sturdy and efficient manner. In other words, rim 144 of a venetian-blind element must be strongly attached to support element 146 and have good electrical contact therewith.
The typical practice is to use venetian-blind elements of copper-beryllium alloy (Cu-Be) with a specially treated active surface. Unfortunately, because of the high conductivity of copper in the alloy, it is virtually impossible to spot weld each venetian-blind structure to support element 146 typically made of Kovar(TM). A pragmatic solution, illustrated in the partial cross-sectional view of FIG. 7, is to provide, for example, a venetian-blind element with blind portions 140 and a relatively flat circult annular rim 152. Support flange 146 is provided with an L-shaped cross-section rim having a cylindrical portion 156 and an annular flat ring portion 158, to which is attached a crimping element 160, to receive and crimp in place the flat annular rim portion 152 of the venetian-blind element and a suitable shaped annular rim portion 154 of grid 136. In other words, each venetian-blind element and its associated grid are held in place within the photomultiplier tube by the crimping action of crimping element 160. Unfortunately, for photomultiplier tube usages that involve high vibration and high temperature environments, this method is unsatisfactory.
A casual review of these problems might suggest that the crimping element be spot welded into attachment with support element 146 and that the grid, likewise, be spot welded to its associated venetian-blind element. Unfortunately, the most suitable material for forming dynodes of the venetian-blind and other types has been found to be a copper-beryllium alloy in which the principal constituent is copper. While this alloy is good as a secondary electron emitter, and thus for dynode structures, the predominance of copper in the alloy causes the same to have a very high conductivity. This makes it virtually impossible to spot weld such elements in place. A highly suitable material for the grids, e.g., 136 in FIGS. 4 and 7, and focusing electrodes such as 28 in FIG. 1, is nickel.
The only use of nickel, for forming a dynode element is suggested in U.S.S.R. Patent No. SU455397, for a device that apparently determines the shape of an x-ray emitting surface (mirror) and eliminates the afocal emission of the radiation characteristics, in which a dynode is made from two different materials. The central patch of the dynode surface is made from a material the specific emission characteristics of which are to be used in the materials structure analysis, e.g., molybdenum or silver, and a surrounding ring portion is made from a material which emits significantly longer wavelength and is easily alternated, e.g., iron or copper. For soft x-rays, the central patch in this device could be made of chromimum, iron, cobalt, nickel or copper, while the outside ring could be of beryllium or aluminum. The function of this device apparently requires that individual specific elements be used for the central patch and the surrounding ring.
There is, therefore, a need for a photomultiplier dynode structure which allows the designer to spot weld individual dynode surfaces for proper affixation of the dynodes for high quantum efficiency, i.e., the ability to provide high secondary emissions.
Apart from the weldability desired in the dynode material, since the first dynode is most important to photomultiplier resolution, it is important that at least the first two dynodes be carefully shaped and placed with respect to each other and successive dynodes for optimum performance thereof. There is, therefore, also a need for a combination of dynode surface geometries in a photomultiplier tube to provide high resolution with relatively weak input from a scintillator under conditions of high vibrational inputs and high temperature.